Description

The nextafter(), nextafterf(), and nextafterl() functions compute the next representable floating-point value
following x in the direction of y. Thus, if y is less
than x, nextafter() returns the largest representable floating-point number less than x.
The nextafter(), nextafterf(), and nextafterl() functions return y if x equals y.

The nexttoward(), nexttowardf(), and nexttowardl() functions are equivalent to the corresponding nextafter()
functions, except that the second parameter has type long double and the functions
return y converted to the type of the function if x equals
y.

Return Values

Upon successful completion, these functions return the next representable floating-point value following
x in the direction of y.

If x == y, y (of the type x) is returned.

If x is finite and the correct function value would overflow, a
range error occurs and ±HUGE_VAL, ±HUGE_VALF, and ±HUGE_VALL (with the same sign
as x) is returned as appropriate for the return type of the function.

If x or y is NaN, a NaN is returned.

If x != y and the correct function value is subnormal, zero,
or underflows, a range error occurs and either the correct function value
(if representable) or 0.0 is returned.

Errors

These functions will fail if:

Range Error

The correct value overflows.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the overflow floating-point exception is raised.

The nextafter() function sets errno to ERANGE if the correct value would overflow.

Range Error

The correct value underflows.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the underflow floating-point exception is raised.

Usage

An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling
these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is
non-zero, an exception has been raised. An application should either examine the
return value or check the floating point exception flags to detect exceptions.

An application can also set errno to 0 before calling nextafter(). On
return, if errno is non-zero, an error has occurred. The nextafterf(), nextafterl().
nexttoward(), nexttowardf(), and nexttowardl() functions do not set errno.